Rat mammary tumor MTW9 can be grown in rats coimplanted with a mammosomatotropic tumor MtTW10 which secretes a factor which prevents ovariectomy-induced regression (OIR). We called this factor galactomedin. The factor is not prolactin, GH, ACTH or any other known steroid. An assay for this activity is possible based upon prevention of ovariectomy induced regression in the hormonally responsive rat tumor-MTW9-D, by injection of serum from MtTW10 bearing rat into ovariectomized-MTW9-rats. Basic extracts from MtTW10 tumor inhibit the growth of MTW9 cell cultures. If this inhibition represents the putative galactomedin the in vitro assay will be used in an effort to isolate galactomedin; if the activity is not identical, the in vivo activity, though expensive and slow, is adequate for the isolation of galactomedin.